This invention relates to a serrodyne generator and more particularly a serrodyne generator providing a constant amplitude serrodyne signal.
It is often necessary in electronic countermeasures systems to frequency modulate the output signal in such a way that the frequency variation is similar to that caused by the doppler effect. A serrodyne circuit is used to accomplish this by varying the transit time of the signal in the output amplifier (traveling wave tube). The transit time is a function of the helix voltage of the traveling wave tube (TWT) and thus by varying the helix voltage, a frequency shift can be produced. The serrodyne circuit must generate a constant peak amplitude, variable frequency sawtooth wave in order to produce the desired frequency shift.
In the past, the serrodyne circuit consisted of a current source to linearily charge a capacitor and a unijunction transistor to set a threshold level and discharge the capacitor. The current source was varied in order to charge the sawtooth frequency. This proved satisfactory at frequencies up to 10kc, however, due to the delay time of the unijunction, at frequencies above 10kc, a variation in amplitude occurs as the frequency is swept.
In order to generate a serrodyne signal at frequencies above 10kc, the present invention was devised in which a threshold circuit is not used to control the sawtooth amplitude and the frequency is independent of current source.